Conventionally, metallic materials and plastic products coated or surface-treated for maintaining the quality or designs are used for home appliances and building materials used by us. While most of stains that are adhered on the metallic materials and the plastic products can be removed by using household or industrial cleaner, it may take time to remove the stains depending on the types of the stains, and in some cases, there is a possibility of decreasing the quality and designs of the home appliances and building materials.
Methods for easily removing the adhered strains include a method of providing a condition in which stains themselves are unlikely to be adhered on the surface of a coating film, such as fluorine resin coatings, and a method of degrading stains adhered on the surface of a coating film by photocatalytic power of titanium oxide, such as photocatalyst coatings. However, the coatings that utilize a water repellent effect, such as fluorine resin coatings are less effective for oily contaminants, and also low in sustaining self-cleaning ability. In addition, the photocatalytic coatings have an ability to decompose an organic substance and remove the stain through excitation light, but fail to develop the ability, due to insufficient strength of the coatings themselves, or ever unless the coatings are irradiated with light. Moreover, these coatings are expensive as compared with conventionally used coatings, thus often limited in intended application.
While methods for easily removing adhered stains other than these methods include a method of making a surface hydrophilic, thereby making adhered stains thereon likely to be removed along with water, there is generally a problem of difficulty in maintaining hydrophilicity over a long period of time.
In order to solve such problems, various methods have been proposed. For example, Patent Literature 1 proposes an antifouling coating liquid containing inorganic oxide microparticles and a surfactant dispersed in a volatile solvent. Coating films obtained by using the antifouling coating liquid have favorable hydrophilicity immediately after the formation of the coating films, but have problems with sustaining hydrophilicity because the surfactant is washed away along with water over time. In addition, the treatment liquid is based on a solvent composed of the volatile solvent, and also not preferable in terms of VOC regulations, working environment, and environmental problem.
Patent Literature 2 proposes an inorganic coating composition containing alumina particles of 20 to 400 nm in average particle size, a silicate oligomer with 95% or more of alkali metals being removed therefrom, or colloidal silica obtained by aging of the oligomer, a surfactant, an organic solvent that is capable of swelling or dissolving an organic base material, and water. Coating films obtained by using the inorganic coating composition have favorable hydrophilicity because of containing the alumina particles, but have difficulty in, with regard to stains due to penetrative oil and the like, removal itself of the stains because the oil penetrates into the alumina particles, and have the problem of generating an odor specific to the inorganic coating films, because the colloidal silica is combined without any modification treatment.
Patent Literature 3 proposes a hydrophilic coating agent containing colloidal silica sol, an acrylic polymer having active hydrogen, a reactive coupling agent, and a curing agent for resin, and furthermore, Patent Literature 4 proposes a hydrophilic coating agent containing colloidal silica sol, an acrylic polymer having active hydrogen, a silane coupling agent, poly(lactone)polyol, a surfactant having active hydrogen, and a curing agent. Coating films obtained by using the hydrophilic coating agents have high hardness and excellent abrasion resistance, but have problems with sustaining hydrophilicity, with a solvent, and also with insufficient liquid stability.
Patent Literature 5 proposes a hydrophilizing agent containing a silicate of a metal selected from alkali metals and alkaline-earth metals, PVA, and an acrylic resin. Patent Literature 6 proposes a hydrophilizing agent containing colloidal silica, PVA, and an acrylic resin which is neutralized with an alkali metal or an alkaline-earth metal to form a salt. These hydrophilizing agents have excellent hydrophilicity and corrosion resistance, but fail to develop a self-cleaning ability against adhered stains over a long period of time.
Patent Literature 7 proposes an aqueous coating hydrophilizing agent characterized in that a thermosensitive polymer that has a phase transition temperature for reversibly switching between hydrophilicity and hydrophobicity is supported on silica particles. Coating films obtained by using the aqueous coating hydrophilizing agent provide hydrophilicity over a long period of time, and however, since coating films are formed by adding silica, which has large size in diameter, into resins, oil stains and so on having permeability are likely to penetrate into the coating films, which results in failing to provide the self-cleaning ability that the inventors aim for.